What is the difference between LXDE and LightDM?
 

I have read https://wiki.debian.org/LightDM and https://wiki.debian.org/LXDE,still confused.
LXDE is one of the DesktopEnvironment options，LightDM is a cross-desktop Display Manager.
What is the difference between LXDE and LightDM?

Lightdm is a single program, a display manager. It does exactly the same job for LXDE as gdm does for gnome and kdm for KDE.

The job of a display manager is to start X and put up a greeter widget with boxes for the user to log on with name and password. It also coordinates all the security and logging work associated with a user login.

LXDE contain many programs,please list some of them.
Must LXDE contain Lightdm as display manager?
Can i delete Lightdm and install other display managers(gdm or kdm) in LXDE?

LXDE is just a desktop to display windows. To provide a means for the user to interact with the software he or she has installed on the hard drive, or medium that holds the software, ie USB Stick etc. , GNU is just software, Linux is just the Kernel that "runs it all". you'd have to look into what Distro you're wanting to use. Going to the source to find out would be best. Why would you have others do all of your work for you?
and
it does not matter what is or is not installed on your Distro when you install it,for the most part, if you do not want it or need it you can always un-install it, then install what it is you do need or want installed. All depending on what that Distro's repo has in it? No, not actually. Source code eliminates that.

No!
Yes, But it'd be better to just - un-install it. then install the display/login manager of your choice, but you'd better be sure to have a good idea on how to set it up. ie. edit its config files to your liking and needs.